Field of the Invention
The present invention is directed towards a process for manufacturing a crystalline alkali metal salt of the general formula (I)[R1—CH(COO)—N(CH2—COO)2]M13  (I)
wherein
M1 is selected from alkali metal cations, same or different,
R1 is selected from C1-C4-alkyl and CH2CH2COOM1,
comprising the step of
(b) crystallizing said alkali metal salt from an aqueous solution containing in the range of from 5 to 30% by weight of alkali metal hydroxide, referring to said aqueous solution.
Furthermore, the present invention is directed towards a crystalline alkali metal salt, and to its use.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Chelating agents of the aminocarboxylate type such as methyl glycine diacetic acid (MGDA) and glutamic acid diacetic acid (GLDA) and their respective alkali metal salts are useful sequestrants for alkaline earth metal ions such as Ca2+ and Mg2+. A lot of aminocarboxylates show good biodegradability and are thus environmentally friendly. For that reason, they are recommended and used for various purposes such as laundry detergents and for automatic dishwashing (ADW) formulations, in particular for so-called phosphate-free laundry detergents and phosphate-free ADW formulations.
A general problem of many aminocarboxylates is their hygroscopicity. In solid formulations such as granules or powders, said hygroscopicity may lead to a reduction or even a complete loss of flowability. Aminocarboxylates will then easily form a sticky mass. In tablets or other solid ready-to-use formulations, said hygroscopicity may lead to reactions of aminocarboxylate with other components of said formulation, or to reactions of other components among each other, for example of peroxide or percarbonate with enzyme. Such reactions are highly undesirable and may lead to a partial or complete deactivation of components of a solid formulation.
The problem of hygroscopicity has been addressed by various authors. In EP 0 845 456 A, an aqueous water-containing melt of MGDA is allowed to solidify. A crystalline mass is obtained. However, it is tedious to remove such mass from the crystallizing vessel, and the degree of crystallinity leaves room for improvement. In WO 2012/150155, comparably enantiomerically pure samples of L-MGDA-Na3 were synthesized. The crystallization of such enantiomerically pure MGDA yields much better crystals but the synthesis is tedious.
It was therefore an objective of the present invention to provide environmentally friendly complexing agents that show a reduced hygroscopicity as judged from their crystallinity, and improved stability towards percarbonate. It was further an objective of the present invention to provide a process for making such complexing agents, and it was an objective to provide applications of such complexing agents.